1. Field of the Invention
This invention relates to spectrometers and, more particularly, to measurements (i.e., metrology) within spectrometers.
2. Description of Related Art
A spectrometer may be considered as an instrument for measuring the positions of spectral lines (e.g., isolated peaks of intensity) in a spectrum of radiation. Spectrometers may be utilized in land, sea, air, and space-based configurations to measure radiance from a source or a field of view in many different spectral channels (i.e., bands within a wavelength region). Typically, a larger number of spectral channels is desirable, because more precision and information about a given spectrum may be obtained. For example, it may be desirable to have tens, or hundreds, or thousands of channels within a given wavelength region (i.e., 8-12 xcexcm, known as the long wave infrared (LWIR) region).
One exemplary type of spectrometer is a Fourier transform infrared (FTIR) spectrometer based on a Michelson interferometer configuration. The Michelson interferometer may be utilized for precise measurements of wavelength or energy distribution in a heterogenous beam of radiation based on an interference pattern. The principles of Michelson two beam interferometry may be utilized for spectroscopy, for example, in the infrared wavelength region. In Michelson spectroscopy use, the large luminosity of the Michelson-based interferometer may be advantageously combined with a photographic spectrograph to simultaneously observe an entire spectrum. The Michelson FTIR spectrometer detects and records a Fourier transform of the desired spectrum, which may be obtained by an inverse transform.
In the Michelson FTIR spectrometer, a laser is typically used as a metrology source for the spectrometer. Such a metrology source may be thought of as a source of radiation that is used in calibrating and ensuring accuracy of the spectrometer. Knowledge of the exact wavelength of the laser source permits proper scaling of infrared power spectral density (IR PSD) data obtained by the FTIR spectrometer. Different approaches may be taken to measure the wavelength of the metrology laser.
One method is to use the IR PSD and to look for the presence of spectral lines of known gas lines. With knowledge of the wavelength for these known gas lines, it is possible to deduct the wavelength of the metrology laser and to calculate the IR PSD. The resolution attainable with this measurement approach is dependent on the gas pressure and the resolution of the instrument. In some field situations, this approach is not suitable. Another approach employs making a calibration measurement using the spectrometer. Using a pointing mirror and by looking into a gas cell or at a lamp (e.g., a neon lamp), the acquired spectrum can be used to deduct the wavelength of the metrology laser and used to calibrate subsequent measurements. However, this approach requires the presence of a gas cell or lamp external to the spectrometer, which may not always be feasible.
Thus, there is a need in the art to accurately calibrate spectrometers utilizing metrology sources that do not unduly add to the complexity of the spectrometers.
Systems and processes consistent with the principles of the invention may, among other things, allow precise measurement of lengths and calibration of spectral data obtained from interferometers and spectrometers using an amplified metrology source.
In accordance with one purpose of the invention as embodied and broadly described herein, a method for measuring length in an interferometer may include generating radiation having a known wavelength profile and amplifying the radiation to produce amplified radiation. The method may also include producing an interference pattern and measuring the interference pattern. One or more lengths within the interferometer may be calculated using the measured interference pattern.
In another implementation consistent with principles of the invention, a device may include a radiant source configured to emit radiation and an optical amplifier configured to amply the radiation emitted by the radiant source to produce amplified radiation. At least two it optical elements may be configured to produce an interference pattern from the amplified radiation. A detector may detect the interference pattern and generate data from the interference pattern. A processor may be configured to measure one or more lengths from the data.
In a further implementation consistent with principles of the invention, a method for determining a length in a spectrometer may include generating radiation including a precisely known wavelength and amplifying the radiation to produce amplified radiation. An interference pattern may be created from the amplified radiation. A precision available for a length measurement may be increased. The interference pattern may be detected. The length measurement may be performed from the detected interference pattern.